With 92 percent of the vote counted, the Socialists appeared to fall short of winning the 176 seats needed for an absolute majority, the BBC reported. Prime Minister Jose Luis Rodriguez Zapatero's ruling party was projected to win 168 seats, to 154 for the Popular Party, led by conservative challenger Mariano Rajoy, the report said.
"The Spanish people have spoken clearly and have decided to open a new period without tension, without confrontation," Zapatero told a cheering audience in a victory speech in Madrid.
"I have called the candidate of the Socialist Party and I have wished him luck for the good of Spain," Rajoy said in his concession speech.
Rising inflation and unemployment, as well as a faltering housing market and immigration, were key issues in the election, the BBC said.
The Socialist Party ran on its liberal reform measures of gender-equality laws and same-sex marriages, while the Popular Party stressed immigration.
The run-up to the election was marred by the shooting death Friday of 42-year-old Socialist councilor Isaias Carrasco in the Basque town of Arrasate.